Micro electro-mechanical systems (micromachines or MEMS devices) have a wide variety of applications and are becoming more prevalent in commercial products. MEMS devices can be fabricated by a variety of methods, one of which is the layer-by-layer deposition and patterning of materials on a substrate. In particular, one of these layers can be a sacrificial layer that provides a foundation upon which the MEMS device can be constructed, and which can be removed once the device fabrication is complete.
One example of MEMS fabrication using a sacrificial layer is the deposition of a polysilicon layer on a phosphosilicate glass (PSG) that is removed at the end of the process. More generally, the sacrificial layer can be replaced with other materials such as polyimide, amorphous carbon, silicon, porous silicon, amorphous silicon, polysilicon polycrystalline SiGe, Al, Cu, Mo or other sacrificial material. More generally, the device layer can be a single layer (e.g. polysilicon, Al, Au, Cu, Polyimide, SixNy, silicon) or multilayer composites including metals, dielectric, polymers, and semiconductors (e.g. metal-oxide-metal, oxide-metal-oxide, etc.). Once the device layer is patterned into the desired shape, the sacrificial layer can be removed by an etchant that is appropriate for the sacrificial material being used.
Unfortunately, the use of a sacrificial layer in this way can adversely affect the fabrication and performance of a MEMS device. In particular, methods of fabricating MEMS devices that involve the deposition and removal of a sacrificial layer can produce devices having stress imbalances, low maximum capacitance, low capacitance density, low capacitance ratio, low capacitance change (ΔC) of fixed capacitors, high device curvature, biaxial curvature, high actuation voltage, mechanical instability, and/or other forms of variability in behavior among multiple arrays, dies, wafers, or lots.